The present invention relates to testing devices and more particularly to a general purpose digital logic monitor for use in servicing digital logic circuits.
A number of techniques have been developed for monitoring the operation of digital logic circuits in the course of servicing those circuits. The simplest technique is simply to check voltages one test point at a time to determine whether the binary value of each voltage is what it should be. Since digital information is often processed in parallel, the circuit being monitored may have to be cycled through its operating sequence a number of times for an operator to monitor all of the points required to establish the value of parallel-presented digital information. Consequently, the point-at-a-time technique is slow and inefficient.
In some types of apparatus, notably digital computers, a permanent display panel is built either into a control console for the device or into access doors to the digital electronics. Such permanent displays continually indicate the state of selected test points within the apparatus. While such permanent displays serve their limited purpose quite adequately, they obviously cannot be used to monitor any test points other than those to which they are permanently connected. Moreover, the cost of such displays is considerable since (1) new displays must be designed each time the associated apparatus is redesigned and (2) there must be a complete set of display components for each set of test points being monitored.
Portable or semi-portable monitors have been developed for use in monitoring specific test points within a specific apparatus. Such portable or semi-portable devices operate in basically the same way as built-in displays and are subject to the same drawbacks and limitations.
Portable monitors for testing integrated circuit packages are also known. Such monitors are useful only to persons who are extremely familiar with the configuration of the circuit being tested.
Prior art monitors of the type discussed above are generally passive devices. Passive monitors accept and display digital information at the normal operating rate of the circuit being monitored. Rapidly changing displays in such monitors make it difficult to isolate the digital information as it exists at particular steps in the operating sequence.